Blade mounting assemblies for mounting bulldozer blades to vehicle bodies are known. These often include mechanisms which allow the lift (height of the blade relative to the vehicle), pitch (orientation of the blade about an axis generally normal to the vehicle centre line plane), angle (orientation of the blade about a generally vertical axis) and tilt (orientation of the blade about a generally horizontal axis parallel to the vehicle centre line plane) to be adjusted for particular operations. Military work vehicles generally operate as conventional bulldozers, i.e. clearing debris, filling holes and so on, but in some cases also function as “earth anchors” in which the blade is buried deeply in the ground to provide a secure anchor to allow the vehicle, fitted with a suitable winch, to recover other vehicles stuck in soft terrain.
Since the blade is generally designed to be fitted to heavy vehicles, the assembly may be subjected to very high loads. For example, if the edge of the blade strikes a sufficiently immovable obstacle while bulldozing, the vehicle will be stopped almost instantly. The loads in the assembly in such a situation may be very large compared to normal bulldozing loads.
At the same time, it is also desirable to be able to adjust blade angle, for example when clearing snow from a carriageway to a roadside. In one known arrangement, one or more powered actuators are connected between the blade and a frame in order to rotate the blade relative to the frame. Such arrangements suffer from the disadvantage that when the blade is facing straight ahead, the actuator cylinders are partly extended, usually at their mid-point of travel, and actuators are then further extended and/or contracted to turn the blade left or right from its mid position. The blade is typically held rigidly in the mid position by locking the actuators. Since hydraulic cylinders are usually used as actuators, these are locked by closing hydraulic valves connected to the cylinder ports, preventing oil leaving or entering the cylinder, and thus preventing the cylinders extending or contracting. In order to avoid damage to the cylinders by excess pressure or buckling, hydraulic circuits typically include pressure relief valves which override this lock if a sufficiently high external force results in hydraulic pressures above some set threshold value, for example as a result of the blade edge striking some sufficiently immovable object and if the moving vehicle has sufficient mass and speed.
Such arrangements suffer from the disadvantage that in order to increase the size of load which the assembly can withstand, larger hydraulic cylinders need to be used, which significantly increases the weight and volume of the assembly.